Radiotelephones having a retractable compound antenna are known in the art. Published Canadian Patent Application No. 2,036,677 describes a radiotelephone having an extendible antenna that is capable of receiving signals in both the extended position and retracted position. This application, however, does not address the impedance imbalance of the antenna in its respective extended and retracted positions, or a solution for providing a matching state between the radio transceiver and antenna.
This patent application, as well as U.S. Pat. No. 5,177,492, also describe rod antenna mounting mechanisms attached to the casing of the radiotelephone, which grip the rod antenna to hold it in an extended or retracted position. The compound antennas used in the mechanisms are relatively rigid where a helix joins the rod, because the juncture of rod and helix is also the point that is electrically coupled to the radio feed terminal when the antenna is in the retracted position. Consequently, they have the problem of being easily damaged at the juncture when the radiotelephone is dropped.
Because retractable, compound antennas typically have a long flexible linear antenna element, this element needs to be guided into the radiotelephone. It is known to use a well formed out of the casing to guide the linear antenna element, such as described in U.S. Pat. No. 5,177,492. This technique has the disadvantage of increasing the bulk and weight of the radiotelephone. Another known technique is to attach a straw-like tube to the casing. This technique has the disadvantage of requiring an assembly step specific to the casing, which otherwise would not be required. Another known technique is to attach the straw-like tube to the circuit board that already has many assembly steps associated with it. This technique has the disadvantage of requiring additional fastening components to be added and assembled to the circuit board, specific for the purpose of holding the straw, to an already crowded circuit board.
There are also known radiotelephones, as described in U.S. Pat. No. 5,374,937, having an extendible antenna that uses a single matching circuit and a first feed terminal to couple an inner circuit to the antenna in the extended position, and then bypass the matching circuit in the retracted position by coupling the inner circuit to the antenna in the retracted position by way of a second feed terminal disposed a distance from the first feed terminal. This technique has the disadvantage of requiring two feed terminals. It further has the disadvantage that the antenna portion between the two feed terminals radiates inside the housing, possibly interfering with sensitive electrical components disposed inside the housing.
Radiotelephones having a retractable compound antenna and a variable impedance matching circuit and single feed terminal are known in the art. One example of such known radiotelephones described in U.S. Pat. No. 5,335,368 and another is embodied in Model No. TZ-820B provided by NEC. This latter radiotelephone has a variable impedance matching circuit coupled with antenna, wherein the matching circuit consists of two parallel circuit paths, each path having a unique reactance. These circuit arrangements are shown in FIG. 1. The parallel circuit paths of a matching circuit 10, having reactive components L.sub.1, L.sub.2, and C, are selected in response to the position of an antenna 12 in order to match the impedance of antenna 12 to the impedance of a coaxial connector 14. The selection is made by a double-pole, double-throw switch 16 arranged so that the switch is thrown by lower end of antenna 12 forcing mechanical contacts, S, from a normally open position to a closed position as antenna 12 is moved from the extended position, shown in solid lines, to the retracted position, shown in dotted lines. The antenna 12 is held in its respective positions by a holding member 22 attached to a housing 24 of the radiotelephone, and guided by a conductive tube 20. Holding member 22 further functions to electrically couple the switch to the antenna. In addition to requiring a switch and a multipath matching circuit, a termination impedance 18 coupled between ground and one end of antenna 12, and the ground-coupled conductive tube 20, are necessary to provide the matching in the retracted position.
This matching technique is not suitable for all handheld radiotelephones. For example, it requires additional components in the matching circuit to provide the two connection states for matching; and it requires a relatively bulky switch to make the connection states. These additional components increase the size of the radiotelephone, which can be a handicap as radiotelephones become smaller. Furthermore, the switch increases the probability of a faulty connection in the sensitive transmission and reception signal path. It further has the disadvantage of requiring an additional termination impedance and conductive tube.
A need therefore exists for a handheld radiotelephone that provides a matching state between the transceiver and retractable compound antenna in both its positions, and does so with an elegant design that increases reliability and manufacturability by reducing the number and complexity of components and moving mechanical parts. A need also exists for a handheld radiotelephone that is not easily damaged when dropped at the point where the rod joins the helix.
Furthermore, a need exists for a radiotelephone having a switch that can be activated by the position of the radiotelephone's retractable antenna, and minimizes the addition of mechanical switching parts in the signal path from the antenna to the transceiver.